A class of a large, and growing, number of people, particularly children, who are unable to read or write.
Illiteracy is beginning to take over the United States of America. After seeing all those Hooked On Phonics advertisements, I have concluded that our country is becoming more and more filled with illiterate brats who deserve a good slapping upside the head.
Apparently, illiteracy is so common that there is evidence of illiteracy in China (the world's #2 superpower). Illiterate children is said to be the result of lazy, irresponsible parents, therefore resulting in much lack of education. It's times like this that, although education sucks (because of schools and how boring they are and having to put up with punk-ass bitches with cruel attitudes), it is very necessary.

If you had a kid who couldn't read a word, wouldn't you like to slap him upside the head too?

In some circles, used as a synonym for incompotent in any way, regardless of any reading ability. For example, it could be used to describe a bad driver, someone who sucks at videogames, or someone who fails a test.